Sesquioxidic components of selected Taiwan soils

Twenty two surface soils, representing six parent materials and four soil groups, were sampled in Taiwan for the present study.Except in the soil derived from volcanic ash, the sesquioxidic components and relatively easily extractable Si were present only in small amounts in the NaOAc and H₂O₂- NaOAc extractable fractions. In addition to Fe, both Si and Al were present in the dithionite-citrate-bicarbonate extracts in considerable amounts, indicating that Si and Al were either present separately or in association with Fe in the sesquioxidic fractions of the soils. The amounts of Si, Al and Fe extracted by the dithionite-citrate-bicarbonate and boiling KOH treatments differed from sample to sample, indicating that they are important variables in the nature of the soils studied.The amounts of the ammonium oxalate-extractable Al and Fe, representing the noncrystalline sesquioxidic products of relatively recent weathering, also differed with parent materials and pedogenic processes. The statistical data indicate that the ammonium oxalate-extractable Al and Fe are related to the contents of organic matter but not to acidity and the contents of clay. The percentage distribution of the extractable Al in the > 2 μm fractions of the selected soil samples ranged from 22.0 to 52.5% and that of the extractable Fe from 11.1 to 38.1%, indicating that the active non-crystalline Al and Fe components in the non-clay fractions deserve close attention in the study of pedogenesis and other soil physicochemical reactions in relation to soil fertility and environmental protection.